Method for manufacturing high efficiency light-emitting diodes

ABSTRACT

A method for manufacturing a light-emitting device comprising the steps of: providing a substrate comprising a first surface and a second surface; forming a plurality of cutting lines on the substrate by a laser beam; cleaning the substrate by a chemical solution; and forming a light-emitting stack on an first surface of the substrate after cleaning the substrate.

BACKGROUND

1. Technical Field

The present invention relates to methods for manufacturinglight-emitting diodes and in particular to a method for producing highefficiency light-emitting diodes.

2. Reference to Related Application

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/379,561, entitled “METHOD FOR MANUFACTURING HIGH EFFICIENCYLIGHT-EMITTING DIODES”, filed on Feb. 25, 2009, now pending, which is acontinuation-in-part of U.S. patent application Ser. No. 11/602,199,entitled “METHOD FOR MANUFACTURING HIGH EFFICIENCY LIGHT-EMITTINGDIODES”, filed on Nov. 21, 2006, now U.S. Pat. No. 7,811,845, the entirecontents of which are incorporated herein by reference.

3. Description of the Related Art

Nitride semiconductor light emitting devices have been employed in awide variety of applications, including traffic lights, backlights ofelectronic apparatus, outdoor optical displays, white light illuminationapparatus, UV light apparatus, and data storage apparatus.

In a nitride semiconductor light-emitting device, a structure of thenitride semiconductor light-emitting device includes a substrate, whichcan be made of sapphire or SiC (silicon carbide). In a process of thenitride semiconductor light-emitting device manufacture, the processincludes forming an epitaxial light emitting stack on a substrate wafer,and then cutting the wafer into dice. The wafer can be separated intodice by a number of ways, for example, the wafer can be mechanicallyscribed, such as with a diamond scribe, and then be separated alongscribing lines. However, the hardness of the sapphire or the SiC isgreater than traditional III-V materials (for example, Si or GaAs), andit is difficult and time-consuming to cut the wafer made of sapphire orSiC. Furthermore, the consumption of the diamond is so great that themanufacturing cost is raised.

Another separation method involves scribing the wafer by a high energydensity laser beam ablating the bonds between atoms of the substrate. Inlaser ablation scribing, the scribing temperature has to be high enoughto break the crystal bonds of the substrate and then results in thebuild-up of waste material along the scribing lines. Because the lightemitting from the light-emitting device can be absorbed by the wastematerial, the light extraction efficiency is reduced.

In U.S. Pat. No. 5,631,190, a dry etching method of removing by-productsgenerated from cutting the silicon carbide substrate by the laser lightis disclosed. However, the dry etching method can damage the device andaffect the product characteristics. Besides, the dry etching methodcauses the wafer breakage and reduces the product yield.

In U.S. Pat. No. 6,849,524, a wet etching method of using a KOH cleaningsolution to remove by-products generated from cutting the sapphiresubstrate by the laser light is disclosed. The cleaning solution canremove slag from the cutting and debris from the breaking. This methodapplies a protective coating on the cutting side of the sapphire wafer,and then cleans the sapphire wafer with the cleaning solution. However,the method cannot remove the slag completely and efficiently.

Therefore, how to remove the by-products resulting from the laser beamcutting efficiently to enhance the light extraction of LEDs becomes animportant issue in the industry.

SUMMARY

Accordingly, the present invention is to provide a method of forming alight-emitting device having higher light-emitting efficiency. Asembodied and broadly described herein, the present invention provides amethod for manufacturing a light-emitting device comprising the stepsof: providing a substrate comprising a first surface and a secondsurface; forming a plurality of cutting lines on the substrate by alaser beam; cleaning the substrate by a chemical solution; and forming alight-emitting stack on an first surface of the substrate after cleaningthe substrate.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings are included to provide easy understanding ofthe invention, and are incorporated herein and constitute a part of thisspecification. The drawings illustrate embodiments of the invention and,together with the description, serve to illustrate the principles of theinvention.

FIG. 1A is a schematic cross-sectional view of a light-emitting devicewith by-products deposited in accordance with a first embodiment of thepresent invention.

FIG. 1B is an SEM photograph showing a top-view of the light-emittingdevice with the by-products.

FIG. 1C is an SEM photograph showing a cross-sectional view of thelight-emitting device with the by-products.

FIGS. 2A, 2B, and 2C are SEM photographs of the light-emitting devicewith the by-products.

FIG. 3 is a diagram showing a relationship between by-products cleaningefficiency, cleaning temperature, and cleaning time.

FIGS. 4A and 4B are schematic cross-sectional views of a light-emittingdevice with by-products deposited in accordance with a second embodimentof the present invention.

FIGS. 5A and 5B are schematic cross-sectional views of light emittingdevice followed by chip processing in accordance with the first andsecond embodiments of the present invention.

FIGS. 6A and 6B are cross-sectional views showing the formation of lightemitting device in accordance with the third embodiments of the presentinvention.

FIG. 7 is a top-view illustrating photoresist and cutting lines in chipdefining step in accordance with the one embodiment of the presentinvention.

FIG. 8 is a flow chart showing the steps in accordance with the oneembodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Reference will now be made in detail to the preferred embodiments of thepresent invention, examples of which are illustrated in the accompanyingdrawings. Wherever possible, the same reference numbers are used in thedrawings and the description to refer to the same or like parts.

Referring to FIGS. 1A-1C, the schematic cross-sectional views showing alight-emitting device with by-products deposited in accordance with afirst embodiment of the present invention respectively. The processes ofmanufacturing the light-emitting device comprises disposing alight-emitting stack 12 over an upper surface of a substrate 10 to forma light-emitting unit 100, coating a first protective layer 13 on anupper surface of the light-emitting stack 12. In another embodiment, thelight-emitting stack 12 is trapezoid wherein the upper surface issmaller than the lower surface so the first protective layer 13 iseasier to be coated on the light-emitting stack 12. The lower surface ofthe substrate 10 is cutting by a laser beam of a wavelength less than365 nm to generate cutting lines 101, and then putting thelight-emitting unit 100 with the first protective layer 13 into a warmchemical solution, e.g. an acid solution, for a predetermined period oftime. By-products 11 comprise a first by-product 111 and a secondby-product 112. The first by-product 111 are deposited in the cuttinglines 101 and the second by-product 112 are deposited around the cuttinglines 101 of the lower surface of the substrate 10 wherein the first andthe second by-product 112 contain the debris of laser ablation from thelight-emitting stack 12 and the substrate 10 such as sapphire.

FIG. 1B is an SEM (scanning electron microscope) photograph showing atop-view of the light-emitting device with by-products. One of thecutting lines 101 has an extension direction A-A′ and the other has anextension direction B-B′. There are a lot of by-products 11 depositedaround the cutting lines 101. FIG. 1C is an SEM photograph showing across-sectional view of the light-emitting device with by-products takenby. There are a lot of by-products 11 deposited around the cutting linealong with direction A-A′ and in the cutting line along with directionB-B′.

The compositions of the chemical solution are phosphoric acid (H₃PO₄)and sulfuric acid (H₂SO₄). The preferred cleaning temperature of thechemical solution is higher than the atmospheric temperature. In apreferred embodiment, the temperature of the chemical solution is175˜300□. Furthermore, a second protective layer can be deposited on thelower surface of the substrate 10 in order to avoid any damage caused bythe chemical solution.

FIGS. 2A to 2C are SEM photographs showing the results of severalexperiments with different cleaning temperature and cleaning time.Referring to FIG. 2A, the cutting lines are cleaned by a chemicalsolution in an experiment I. The ratio of the H₃PO₄ to the H₂SO₄ adoptedin experiment I is 2:1, and the temperature of the chemical solution is130□. The light-emitting device 1 is cleaned in the chemical solutionfor 10 minutes, but the first and second by-products 111 and 112 cannotbe removed. Referring to FIG. 2B, the cutting lines are cleaned by achemical solution in an experiment II. The light-emitting device 1 iscleaned in the chemical solution having the temperature of 240□ for 2minutes. The second by-product 112 deposited around the cutting line ofthe lower surface of the substrate 10 are removed, but the firstby-product 111 deposited in the cutting line still cannot be removed.Referring to FIG. 2C, the cutting lines are cleaned by a chemicalsolution in an experiment III. The light-emitting device 1 is cleaned inthe chemical solution of the temperature of 330□ for 1 minute, and thefirst and second by-products 111 and 112 are totally removed.

The foregoing are just some experiments of the first embodiment. Aftermany experiments, the results are plotted in FIG. 3. The curve A is alimit of the cleaning condition to remove the second by-product 112, andthe curve B is a limit of the cleaning condition to remove the firstby-product 111 and the second by-product 112. If a cleaning procedure isperformed with the condition of region I, the by-products cannot beremoved. When a cleaning procedure is performed with the condition ofregion II for which the temperature of the chemical solution is between175□ to 275□, the second by-product 112 can be removed. When a cleaningprocedure is performed with the condition of region III for which thetemperature of the chemical solution is equal or greater than 275□, thefirst by-product 111 and the second by-product 112 can be removed.

The composition ratio of the chemical solution can be changed based onthe user requirement. If the user prefers a shorter cleaning time, achemical solution with a higher ratio of the H₂SO₄ and higher cleaningtemperature can serve the purpose. The chemical solution has a goodselectivity between the by-products 11 and the substrate 10, and most ofthe by-products 11 can be removed without damaging the substrate.

Referring to FIGS. 4A and 4B, schematic cross-sectional views of alight-emitting device with by-products in accordance with a secondembodiment of the present invention is shown. The method ofmanufacturing the light-emitting device comprises disposing alight-emitting stack 22 over an upper surface of a substrate 20 to forma light-emitting unit 200, coating a first protective layer 23 of anexemplary thickness of 1-4 μm on an upper surface of the light-emittingstack 22. Referring to FIG. 4B, in some embodiments, the firstprotective layer 23 is also covering the substrate 20. In anotherembodiment, the light-emitting stack 22 is trapezoid wherein the uppersurface is smaller than the lower surface so the first protective layer23 is easier to be coated on the light-emitting stack 22.

Next, cutting the light-emitting stack 22 from the upper surface by alaser beam with a wavelength less than 365 nm, and putting thelight-emitting unit 200 with the first protective layer 23 into achemical solution for a predetermined period of time to remove theby-products resulted from the laser cutting. When the substrate is cutby the laser beam from the upper surface of the substrate 20, cuttinglines 201 are formed. By-products 21 comprise a third by-product 211 anda forth by-product 212. The third by-product 211 are deposited in thecutting lines 201 and the forth by-product 212 are deposited around thecutting lines 201 wherein the third and the fourth by-products containthe debris of laser ablation from the light-emitting stack 22 and thesubstrate 20.

The composition of the chemical solution includes H₃PO₄ and H₂SO₄. Thepreferred temperature of the chemical solution is higher than theatmospheric temperature. In a preferred embodiment, the temperature ofthe chemical solution is 175˜300□. In another embodiment, thetemperature of the chemical solution is 320□. The light-emitting device2 is cleaned in the chemical solution for 10-60 seconds. A secondprotective layer can be deposited on the lower surface of the substrate20 in order to avoid any damage caused by the chemical solution. Thedepth of the cutting lines 201 can reach downward to about 5%˜70% of thethickness of the substrate 20. For a substrate 20 such as sapphire at athickness of 100˜200 μm, the depth of the cutting lines 201 issubstantially in a range of 5 μm to 140 μm.

Referring to FIGS. 5A and 5B, after removing the by-products, theprotective layers are removed. The light emitting stack 32 includes afirst semiconductor layer 321, a semiconductor light-emitting layer 322,and a second semiconductor layer 323. The semiconductor layers 321, 322,and 323 of the light emitting stack 32 can be grown by epitaxytechnology such as a metal organic chemical vapor deposition method(MOCVD method), a molecular beam epitaxy method (MBE method) or thelike, and with proper doping if necessary. The semiconductorlight-emitting layer 322 and the second semiconductor layer 323 areetched to expose partial of the first semiconductor layer 321.Following, a first electrode 324 and a second electrode 325 are formedon the first semiconductor layer 321 and the second semiconductor layer323 respectively.

Referring to FIGS. 6A and 6B, cross-sectional views of a light-emittingdevice in accordance with a third embodiment of the present invention isshown. To avoid the damage to the light-emitting stack caused by thechemical solution, both the process of cutting the substrate to formcutting lines and the afterward process of putting the substrate into achemical solution to remove the by-product are done before thelight-emitting stack is formed. The cutting lines 401 can be formed onthe upper surface of the substrate 40 as shown in cross-sectional views(a) of FIGS. 6A and 6B, or can be formed on the lower surface of thesubstrate 40 as shown in cross-sectional views (b) of FIGS. 6A and 6B.Here, the cross-sectional views (a) of FIGS. 6A and 6B, where thecutting lines 401 are formed on the upper surface of the substrate 40,are taken as an example for following illustration.

The method of manufacturing the light-emitting device comprises cuttingthe substrate 40 from the upper surface by a laser beam with awavelength less than 365 nm, and putting the substrate 40 in a chemicalsolution for a predetermined period of time to remove the by-productsresulted from the laser cutting. When the substrate is cut by the laserbeam from the upper surface of the substrate 40, cutting lines 401 areformed. By-products 41 comprise a fifth by-product 411 and a sixthby-product 412. The fifth by-product 411 are deposited in the cuttinglines 401 and the sixth by-product 412 are deposited around the cuttinglines 401 wherein the fifth and the sixth by-products contain the debrisof laser ablation from the substrate 40.

The composition of the chemical solution includes H₃PO₄ and H₂SO₄. Thepreferred temperature of the chemical solution is higher than theatmospheric temperature. In a preferred embodiment, the temperature ofthe chemical solution is 175˜300□. In another embodiment, thetemperature of the chemical solution is 320□. The substrate 40 iscleaned in the chemical solution for 10-60 seconds. As illustrated inprevious embodiments, a first protective layer can also be deposited onthe upper surface of the substrate 40 before the cutting process inorder to avoid any damage caused by the chemical solution. Similarly, asecond protective layer can be deposited on the lower surface of thesubstrate 40 in order to avoid any damage caused by the chemicalsolution. After removing the by-products, the protective layers areremoved.

Next, disposing a light-emitting stack 42 over an upper surface of asubstrate 40 to form a light-emitting unit 400. Similar to thosepreviously illustrated in FIGS. 5A and 5B, the light-emitting stack 42may include a first semiconductor layer, a semiconductor light-emittinglayer, and a second semiconductor layer. Because the cutting lines 401comprise a different crystal plane from that of the upper surface of thesubstrate 40, during epitaxial growth of the light-emitting stack 42,the light-emitting stack 42 material does not grow directly on thecutting lines 401. However, by fine tuning the forming parameters, suchas pressure, temperature, and flowing rate of gases to grow thelight-emitting stack 42, the materials of the light-emitting stack 42can still be formed on the cutting lines 401, but as the cutting line401 comprises multiple crystal planes caused by a laser beam,polycrystalline and/or amorphous materials of the light-emitting stack42 are formed directly on the cutting lines 401. In contrast, singlecrystal materials of light-emitting stack 42 are formed on the uppersurface of the substrate 40 other than the cutting lines 401. Thepolycrystalline and/or amorphous materials of the light-emitting stack42 formed directly on the cutting lines 401 are more fragile than thesingle crystal materials of the light-emitting stack 42 on other regionof the upper surface of the substrate 40. Therefore, it is much easierto break the light-emitting stack 42 along the cutting lines 401 thanother region of the substrate 40 when being applied to the samemagnitude of force. A subsequent wafer breaking process can be providedto separate the wafer into chips. As a result, the wafer breakingprocess breaks the fragile materials of the light-emitting stack 42directly on the cutting lines 401, and the broken materials are removedto leave side walls 400 a of light-emitting stack 42 rough. In otherwords, at least one side wall of one of the chips is rough, and thelight extraction efficiency of the light-emitting units 400 is thereforeenhanced.

Still another embodiment is illustrated in FIG. 7, and correspondingprocess is illustrated in FIG. 8. Step 801 is to provide a substrate 40comprising a wafer with a diameter between 2 inches and 8 inches. Step802 is to cut the substrate 40 to form cutting lines 401. As illustratedpreviously, protective layers can also be deposited on one or bothsurfaces of the substrate 40 before the cutting process in order toavoid any damage caused by the chemical solution in a following cleaningstep 803. The cleaning step 803 is mainly to clean the by-products ofthe cutting step 802 by using the chemical solution as illustrated inprevious embodiments. And then step 804 is to form a light-emittingstack 42. The details for these four steps are as the aboveillustration. For either type illustrated above, i.e., whether thelight-emitting stack 42 materials are formed on the cutting lines 401 ornot, a lithography and etch process can be used in a light-emittingstack patterning step 805 to pattern the light-emitting stack 42 into aplurality of light-emitting units 400. Each of the light-emitting units400 substantially corresponds to a chip region 760 previously defined bythe cutting lines 401. As illustrated in FIG. 7, a photoresist is usedto cover the light-emitting stack 42 to the whole wafer 740, and thenthe lithography method is used to align and remove part of photoresistthat is substantially directly over the cutting lines 401 to leave theremaining photoresist 750 to serve as a mask for a subsequent etchprocess. And then the etch method such as dry etch, with the remainingphotoresist 750 as a mask, is used to remove part of the light-emittingstack 42 that is substantially over the cutting lines 401 to expose thecutting lines 401 and thus pattern the light-emitting stack 42 into aplurality of light-emitting units 400. The remaining photoresist 750 isthen removed. The light-emitting unit 400 can be defined, as illustratedin FIG. 7, with a side wall 400 a adjacent to but away from the edges401 a of the cutting lines 401 with a distance D, or can be defined witha side wall 400 a almost close to the edges 401 a of cutting lines 401.That is, the light-emitting stack 42 is patterned into a plurality oflight-emitting units 400, and each of the light-emitting units 400substantially corresponds to the chip region 760. After thelight-emitting stack patterning step 805, a light-emitting devicestructuring step 806 is followed. This is similar to those previouslyillustrated in FIGS. 5A and 5B, where the semiconductor light-emittinglayer and the second semiconductor layer are etched to expose partial ofthe first semiconductor layer. Following, a first electrode and a secondelectrode are formed on the first semiconductor layer and the secondsemiconductor layer respectively.

In one embodiment of the present invention, the method thereof furthercomprises a wafer grinding and/or polishing step 807. The wafer grindingstep is to thin the substrate 40 and the polishing step is to flattenthe substrate 40. And the method thereof further comprises a waferbreaking or dicing step 808 along the cutting lines 401 to separate thewafer into chips.

The material of the substrate comprises at least one material selectedfrom the group consisting of sapphire, SiC, GaN, AlN, ZnO, MgO, and anycombination thereof. The material of the light-emitting stack comprisesat least one material selected from the group consisting of Ga, Al, In,As, P, N, Si, and any combination thereof. The material of the firstprotective layer or the second protective layer comprises at least onematerial selected from the group consisting of gold, platinum, titanium,Ta₂O₄, TiO₂, Al₂O₃, Si, carbon film, Si₃N₄, SiO₂, epoxy, andphotoresist.

In this invention, the light-emitting stack is cutting before the waferpolishing process; it can reduce the breaking rate of the wafer,especially when the thickness of the light-emitting stack is increasing.Besides, by cutting the light-emitting stack before forming theelectrodes and dicing process, the light emitting stack is wellprotected to avoid the chemical solution damaging the semiconductorlayer and its electrical property.

It will be apparent; however, that other variations and modificationsmay be made to the described embodiments, with the attainment of some orall of their advantages. Therefore, it is the object of the appendedclaims to cover all such variations and modifications that fall withinthe spirit and scope of the invention.

1. A method for manufacturing a light-emitting device comprising thesteps of: providing a substrate comprising a first surface and a secondsurface; forming a plurality of cutting lines on the substrate by alaser beam; cleaning the substrate by a chemical solution; and forming alight-emitting stack on the first surface of the substrate aftercleaning the substrate.
 2. The method for manufacturing a light-emittingdevice according to claim 1, wherein cleaning the substrate by achemical solution comprises cleaning the substrate by the chemicalsolution with a temperature not lower than
 175. 3. The method formanufacturing a light-emitting device according to claim 1, wherein thecutting lines defines a plurality of chip regions.
 4. The method formanufacturing a light-emitting device according to claim 3, furthercomprising patterning the light-emitting stack into a plurality oflight-emitting units, wherein each of the light-emitting unitssubstantially corresponds to the chip region.
 5. The method formanufacturing a light-emitting device according to claim 4, furthercomprising: forming two electrodes on each of the light-emitting units;and dicing the substrate with the light-emitting units into a pluralityof separate chips along the cutting lines.
 6. The method formanufacturing a light-emitting device according to claim 5, furthercomprising a grinding step to thin the substrate and/or polishing stepto flatten the substrate before the dicing.
 7. The method formanufacturing a light-emitting device according to claim 1, whereinforming the light-emitting stack comprises: forming a firstsemiconductor layer; forming a semiconductor light-emitting layer overthe first semiconductor layer; and forming a second semiconductor layerover the semiconductor light-emitting layer.
 8. The method formanufacturing a light-emitting device according to claim 1, wherein theplurality of cutting lines are formed on the first surface of thesubstrate.
 9. The method for manufacturing a light-emitting deviceaccording to claim 8, further comprising forming a protective layer overthe first surface of the substrate before forming a plurality of cuttinglines on the substrate by a laser beam.
 10. The method for manufacturinga light-emitting device according to claim 1, wherein the plurality ofcutting lines are formed on the second surface of the substrate.
 11. Themethod for manufacturing a light-emitting device according to claim 10,further comprising forming a protective layer over the substrate beforeforming a plurality of cutting lines on the substrate by a laser beam.12. The method for manufacturing a light-emitting device according toclaim 8, wherein the light-emitting stack is formed only on the firstsurface of the substrate other than the cutting lines.
 13. The methodfor manufacturing a light-emitting device according to claim 1, whereina composition of the chemical solution includes phosphoric acid (H₃PO₄)and sulfuric acid (H₂SO₄).
 14. The method for manufacturing alight-emitting device according to claim 2, wherein the preferredtemperature is 175˜300° C.
 15. The method for manufacturing alight-emitting device according to claim 1, wherein forming thelight-emitting stack on the first surface of the substrate comprisesfine tuning parameters for forming the light-emitting stack to formpolycrystalline and/or amorphous materials of the light-emitting stackdirectly on the cutting lines and form single crystal materials oflight-emitting stack on the first surface of the substrate other thanthe cutting lines.
 16. The method for manufacturing a light-emittingdevice according to claim 15, further comprising a breaking process toseparate the substrate with the light-emitting stack into chips.
 17. Themethod for manufacturing a light-emitting device according to claim 16,wherein at least one side wall of one of the chips is rough.
 18. Themethod for manufacturing a light-emitting device according to claim 1,wherein the cutting lines have a depth of about 5%˜70% of the thicknessof the substrate.
 19. The method for manufacturing a light-emittingdevice according to claim 1, wherein the substrate has a diameterbetween 2 inches and 8 inches.
 20. The method for manufacturing alight-emitting device according to claim 1, wherein forming thelight-emitting stack on the first surface of the substrate comprisesgrowing epitaxial layers on the substrate and cutting lines.